Dynamins 2 and 3 Are Required for Human Megakaryocytes Directional Migration

Abstract:

Megakaryocytes (MKs) undergo directional migration from the proliferative osteoblastic niche within the bone marrow (BM) environment to the capillary-rich vascular niche for platelet production and release into the pulmonary circulation. This process is regulated, in part by dynamins, large GTPase proteins that regulate cellular functions such as endocytosis, vesicle transport and cell migration. Additional functions of dynamins include the formation of actin-rich structures, such as lamellipodia and dorsal membrane ruffles, invadopodia and podosomes. Previous studies have shown that mutations in Dynamin 2 (DNM2) cause thrombocytopenia in humans. To explore the function of dynamins in megakaryocyte migration and platelet production in more depth, we monitored the response of cells to chemotaxis SDF1α gradient signal by a microfluidic device-based approach. We observed an impaired directional migration by both human megakaryocytic cell lines and primary cells treated either with dynasore, a small molecule inhibitor of dynamins, or shRNA knockdown of Dynamin 2 and 3 (DNM2, DNM3). Since directional cell migration is tightly regulated by actin cytoskeletal rearrangements, we next measured actin polymerization and RhoA activity. We observed a profound decrease in the F-actin and Rho GTPase activity upon loss of DNM2 and DNM3 function. Next, since the response to chemoattractant signal is navigated by SDF1 through its receptor CXCR4, we explored the CXCR4 receptor response to ligand in dynamin defective megakaryocytes. Interestingly we observed an increase in CXCR4 expression in the dynasore treated primary human cells. Additionally, combined inhibition of DNM2 and DNM3 or over expression of dominant negative Dnm2-K44A or GTPase-defective DNM3 decreased the active β1- integrin (ITGB1) activity, which indicates a decrease in the integrin mediated endo/exocytic cycling during cell migration. Finally, to understand the role of dynamin in endosome recycling, we assayed the distribution of Rab11, a marker of recycling endosomes. We noticed an abnormal clustered staining pattern of Rab11 in dynasore-treated MKs which is indicative of a disruption in recycling pathways. This observation suggests decreased recruitment of the recycling pathway in dynasore-treated cells. Altogether, in this study we demonstrate that dynamins regulate MKs directional migration towards the SDF1α chemotaxis signal in the bone marrow and governs endocytosis and cell receptor trafficking.